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VIRUSES video what are they? are they living or nonliving? “viruses lead a kind of borrowed life” - researches history… Adolf Mayer • 1883 • disease of tobacco leaves (mosaic color) could be transferred rubbing diseased leaves onto healthy ones • cause invisible under a microscope • maybe tiny bacteria? Dimitri Ivankowsky • a decade later • filtered out bacteria • disease still passed • maybe bacteria too small (passed through the filter) or produced toxins? Martinus Beijerinck • infectious agent in the sap could reproduce • could NOT be cultivated on nutrient media • must be something much smaller than a bacterium (hmmm… what can it be? Wendell Stanley • 1935 • crystallized the infectious particle • tobacco mosaic virus structure of viruses (nucleic acid enclosed in a protein coat, some have a membranous envelope) and they are not… good for us viral genome viruses can posses… •double stranded DNA •single stranded DNA •double stranded RNA •single stranded RNA usually a single linear or circular molecule of nucleic acid capsids and envelopes capsid • Protein shell • Protein subunits capsomeres • Various shapes envelopes • only some viruses (e.g. influenza) • derived from the host cells • but also proteins and glycoproteins of viral origin Bacteriophages (some of the most complex and studied viruses) need a host to reproduce (obligate intracellular parasites) narrow or broad host range in general… • virus delivers its genome inside the host cell • the host provides nucleotides, and all the components needed to make viral protein (enzymes, tRNA, ribosomes, ATP, etc…) • the simplest type of viral reproductive cycle ends with the exit of a large number of viruses simplified viral reproduction the lytic cycle • ends with death of the host cell (lysis) • virulent phages how do bacteria fight back? • mutations causing unrecognizable receptor are selected for • the viral (foreign) DNA is often cut up by restriction enzymes Sometimes phages coexist with their hosts… the lysogenic cycle • does not destroy the host cell • viral DNA is incorporated into the host’s DNA (prophage) • the viral DNA replicates every time the cell replicates • bacteriophages using both modes of reproduction – temperate phages various viral genes are being expressed, changing the phenotype of the host cell (often leading to formation of more harmful bacteria) An environmental signal usually triggers a switchover to the lytic mode reproductive cycles of animal viruses • numerous variations, mostly dependent on the type of the viral genome… • DNA ? • RNA? • double stranded? • single stranded? viral envelopes • used to enter the host cell • mostly derived from the host’s plasma membrane RNA as viral genetic material • mostly animals infecting viruses • class IV – directly used as mRNA • class V – RNA serves as a template for mRNA • class VI – retroviruses RNA DNA (reversed transcriptase) e.g. HIV (like other retroviruses – envelope and two molecules of single-stranded RNA) HIV video evolution of viruses • because they depend on cells, most likely evolved after first cells • most likely from naked bits of cellular nucleic acids that moved from one cell to another • two main candidates: plasmids and transposons the interesting case of mimivirus • described in 1992 • at the time the largest known virus (over 400nm in diameter) • genome 1.2 M bp • 911 protein coding genes • it blurs boundaries between viruses and the smallest parasitic cellular organisms viral diseases in animals vaccine - a harmless variant or derivate of a pathogen that stimulates the IS smallpox – successfully eradicated in 1979 (a vaccination initiative by the WHO) emerging viruses – appear suddenly or are new to medical scientists • HIV • Ebola virus • West Nile virus • SARS how do they just show up? (3 reasons) • unusually high rate of mutation • dissemination from a small isolated human population • spread of existing viruses from other animals “Spanish flu” • 1918 – 1919 pandemic • Influenza virus type A • the source: most likely birds • Infected about 500M people (killed 50-100M) Different strain of influenza A are given standardized names… e.g. (the “Spanish flu” strain) the “avian flu” strain viral diseases in plants • more than 200 types • $15B estimated annual loss in crop destruction • similar basic structure and reproduction cycle as animal viruses • horizontal transmission • vertical transmission viroids • circular RNA molecules • only a few hundreds of nucleotides long • do not encode proteins • affects plants • siRNA (RNA silencing) Prions (the scariest of all) • infectious agent composed only of (misfolded) protein • causing TSEs – neurodegenerative disease of the CNS (formation of amyloids) • ability to misfold other proteins • A long incubation period (5-20 years) • Nearly impossible to denature • Most known diseases include: BSE (aka mad cow disease) and CJD in humans